The present invention relates to a magnetic recording medium having a ferromagnetic metal thin film as a magnetic layer. More particularly, the present invention relates to a ferromagnetic metal thin film type magnetic recording medium which is made excellent in running properties, durability and shelf stability by specifying the structure and composition of a lubricant which is to be present on a protective film formed on the magnetic layer.
Magnetic recording mediums, e.g. magnetic tapes, floppy disks, etc., are commonly coated at the magnetic layer surface thereof with a lubricant to improve lubrication between the magnetic recording medium and the magnetic head and to enhance the running durability of the magnetic recording medium.
With the achievement of high recording density, the magnetic layer surfaces of magnetic recording mediums have become smoother. Further, magnetic recording mediums are used under various environmental conditions, and information recorded thereon are usually stored for a long period of time ranging from several years to several tens of years. Accordingly, magnetic recording mediums are demanded to have running durability and shelf stability under a wide variety of environmental conditions. Under these circumstances, conventional lubricants have become unable to exhibit satisfactory effect.
Further, small-sized video tape recorders such as cam coders, i.e. video tape recorders having a camera integrated therewith, and 8-mm video tape recorders are often used outdoor. Therefore, magnetic recording mediums are demanded to endure use in a wide range of environmental conditions.
To improve the density of magnetic recording on video tapes and magnetic disks in conformity to future developments of high-definition television, digital recording, etc., metal thin film type magnetic recording mediums which use a ferromagnetic metal thin film as a magnetic layer, and which are useful for high-density recording have been expected. However, in a metal thin film type magnetic recording medium, the magnetic layer is protected by only an extremely thin metallic oxide layer. Therefore, it has been demanded to provide a lubricant layer material which is capable of not only ensuring running properties and durability but also markedly improving shelf stability of ferromagnetic metal thin film type magnetic recording mediums.
In particular, ferromagnetic metal thin film type magnetic recording mediums are being improved so as to enable achievement of higher-density recording by smoothing the surface and also changing the composition of the magnetic layer from CoNi--O to Co--O or Co--Fe that contains Co--O, etc. so as to attain high Bm. With the conventional metal thin film type magnetic recording mediums that have a high cobalt content, however, it has heretofore been difficult to obtain stable running properties, favorable durability and corrosion resistance.
Hydrocarbon- or fluorine-containing lubricants which have various structures, and which are excellent in lubricating performance have been examined as being lubricants for metal thin film type magnetic recording mediums, and it is known that an organic fluorine compound having a hydrophilic functional group in a molecule thereof is capable of improving running durability to a considerable extent.
However, even if a lubricant having excellent characteristics is used, a smooth metal thin film type magnetic recording medium having improved electromagnetic transducing characteristics suffers from the problem that the lubricant attached to the surface of the magnetic layer is gradually lost because of repeated sliding movement, resulting in deterioration of the characteristics.
One approach that has heretofore been made to improve the characteristics is to use a lubricant having various polar groups in a molecule thereof. For example, Japanese Patent Application Unexamined Publication Number hereinafter referred to as "JP(A)"! 59-119537 and Japanese Patent Application Post-Examination Publication Number hereinafter referred to as "JP(B)"! 4-50644 disclose a technique in which a carboxylic acid, an ester, a phosphoric ester, etc. are used. JP(A) 4-274017 and 4-372716 disclose the use of an alkylamine salt of a carboxylic acid and an alkylamine salt of phosphonic acid. However, the disclosed techniques involve the problem that repeated running durability at low temperature is inferior. Accordingly, there have also been made many studies to improve the characteristics by using two or more different kinds of lubricant in combination.
JP(A) 62-141625 discloses the use of a carboxylic acid and a fluorine-containing ester in combination. With this method, the repeated running durability at low temperature can be markedly improved in comparison to a case where each lubricant is used alone. However, the magnetic recording medium using a carboxylic acid and a fluorine-containing ester in combination has the problem that it is inferior in corrosion resistance, particularly shelf stability under high-temperature and high-humidity environmental conditions.
Further, JP(A) 4-205712 proposes a method in which a lubricant layer is formed from two different kinds of lubricant, and in which the amounts of lubricant at the magnetic layer side and at the back coat layer side are specified. With this method, however, no satisfactory lubricating properties can be obtained.
JP(A) 57-29767, 58-188326, 60-63711, 60-63712, 62-209718 and 01-211215 disclose a method in which a lubricant is applied to a back coat layer provided on the side of a magnetic recording medium which is reverse to the side thereof on which a magnetic layer is formed, thereby enabling the lubricant to be appropriately supplied from the back coat layer to the magnetic layer surface where the lubricant is gradually lost by sliding movement. Such a coating method enables an improvement in repeated running durability. However, when a lubricant is coated on only the back coat layer, durability is difficult to ensure; when a lubricant is coated on both the back coat layer and the magnetic layer, a surplus of lubricant is inevitably present on the magnetic layer. Consequently, the coefficient of static friction rises, giving rise to problems such as undesired sticking of the magnetic recording medium.
In terms of corrosion resistance, which is a problem in practical use of magnetic recording mediums that use a ferromagnetic metal thin film as a magnetic layer, the required corrosion resistance cannot sufficiently be ensured by only a fluorine-containing lubricant having a polar group in a molecule thereof. Accordingly, it has been proposed to use such a lubricant in combination with a rust preventive. However, it has heretofore been difficult to ensure the required corrosion resistance by the combined use of a lubricant and a rust preventive that are selected from those which are generally known.
Under these circumstances, attempts have recently been made to improve durability and corrosion resistance by providing a protective film on the magnetic layer of a magnetic tape as well as a magnetic disk. Among various protective films, a carbon film, which is represented by a diamond-like carbon film, has become of major interest as a protect film because it has a high hardness and is unlikely to seize to a sliding member under all environmental conditions. However, when such a carbon protective film is used alone, the coefficient of friction is raised by repeated sliding movement, resulting in breakage of the film. Therefore, it is a common practice to coat a lubricant having a polar group on the protective film. For example, JP(A) 5-151559 discloses a magnetic recording medium in which a long-chain alkylsilane compound is applied to the surface of a carbon protective film. However, under low-temperature and low-humidity environmental conditions, the required lubricating performance cannot satisfactorily be ensured by only a long-chain alkylsilane compound, and it is likely that the protective film will be broken as a result of the rise of the friction coefficient.
Thus, it has been demanded to develop a magnetic recording medium which has the coefficient of friction stabilized at a low level, and which is excellent in still durability, repeated running durability and corrosion resistance and capable of high-density magnetic recording in particular.